The present invention relates to a heat treatment furnace.
Electrically heated furnaces have several advantages over gas heated furnaces. They are environmentally perferable, since they cause less noise, lower surrounding temperatures and less pollution. Moreover, they have advantages in operation, such as less complicated temperature controls, reduced heat loss, reduced wear characteristics and more reliable energy supply.
In heat treating steel, it is frequently necessary to use a protective gas, e.g., in the case of carburetting and dry-cyaniding processes. The presence of such a protective gas, however, places great demands on the electrical heating elements. It is therefore known in the art to use as heating elements metallic, electrical resistance wires wound upon a ceramic core and normally enclosed in metallic and ceramic pipes or sheaths, whereby the heating elements are protected from the furnace atmosphere, and their useful life is extended to a commercially acceptable extent.
However, the use of protective sheaths has its disadvantages. Manufacturing and maintenance costs are high, and performance of the elements is limited to the avoidance of overheating. Heating elements of this type also have an unavoidable thermal sluggishness which causes extended heating time and difficulties in maintaining precise temperature control and distribution in the furnace.
In the case of continuous operation, the performance and temperature of the elements and their protective sheaths in the feed zone of the furnace are maximal when the steel is preheated or heated to a predetermined treatment temperature. In the case of intermittent operation, they are maximal during heating of the charge to the treatment temperature. Thus, the heating elements are exposed to particularly great stresses in the said feed zone and/or heating period.